This invention relates generally to the field of refrigerant cooled variable frequency (VFD) drives associated with a refrigerant compressor, and more particularly to a method and system which enhances the transmitted power of the VFD at optimal operating conditions, while restricting the transmitted power when conditions are not ideal for cooling.
Compressors used in many refrigeration systems generally require close control over the compressor motor speed in order to maintain the system within desired limits under varying load conditions. The compressors are therefore equipped with variable frequency drives (VFD) that contain power electronic components that can overheat and thereafter require cooling. Cooling is generally provided to the power electronics by mounting the electronics on a heat sink and removing the heat from the sink by circulating coolant in or around the heat sink. The capability of the heat sink and cooling system are of primary consideration in determining the power capacity of the VFD.
VFD""s that are used today are designed and rated for the worst case ambient operating conditions. By measuring the temperature of the VFD electronics base plate, a control device can make a decision as to the power handing capability of the drive. Thus, below a certain nominal VFD heat sink temperature, the drive is rated to operate at a specified temperature. When the temperature of the heat sink goes above the nominal operating temperature, the drive power handling rating is reduced, thus limiting the cooling capacity of the chiller. This operating condition is typically an off-design condition where the chiller is not rated for a particular capacity.
U.S. Pat. No. 6,116,040 describes a refrigerant system in which the VFD electronic components are mounted on a heat sink such that refrigerant from the system condenser is passed through the heat sink by a flow line and returned to the low pressure side of the system. A control valve is mounted in the flow line which throttles refrigerant passing through the line to produce cooling of the heat sink to maintain the temperature of the electronic components within a desired range. A temperature sensor on the heat sink provides temperature information to the control valve.
Briefly stated, a control system for a refrigerant cooled variable frequency drive (VFD) for a chiller includes a controller which controls an electronic expansion valve to attempt to maintain a heat sink for the variable frequency drive within its design operating range. When the controller is unable to maintain the heat sink within its design operating range, a load on the chiller is reduced, either by reducing the frequency of the variable frequency drive, or if the chiller includes a compressor with internal guide vanes, by partially closing the guide vanes. The power rating of the VFD can thus be optimized based on its heat generating components operating at a controlled condition, unlike an air-cooled device which is dependent on the ambient environment.
According to an embodiment of the invention, a control system for a refrigerant cooled variable frequency drive for a chiller includes first control means for controlling an electronic expansion valve to attempt to maintain a heat sink for the variable frequency drive within its design operating range; and load reduction means for reducing a load on the chiller when the first control means is unable to maintain the heat sink within its design operating range.
According to an embodiment of the invention, a method for controlling a refrigerant cooled variable frequency drive for a chiller includes the steps of controlling an electronic expansion valve to attempt to maintain a heat sink for the variable frequency drive within its design operating range; and reducing a load on the chiller when the step of controlling is unable to maintain the heat sink within its design operating range.